1. Field of the Invention The present invention relates to a voltage controlled oscillator (VCO) using a differential amplifier, and more particularly to a VCO using LC oscillator and a differential oscillator. The present application is based on Korean Application No. 2001-72471, which is incorporated herein by reference.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a conventional VCO. It utilizes a differential amplifier and an LC tank(oscillator).
Referring to FIG. 1, a differential amplifier includes a couple of MOSFETs 10 and 12 for embodying a negative resistance, and another MOSFET 36 for embodying a current source controlling the MOSFETs 10 and 12. An LC tank includes variable capacitors 26 and 28 and inductors 14 and 20 which are connected to a drain port of each MOSFET 10 and 12. A current source 36 is controlled by a voltage applied to an input node 38. Variable capacitors 26 and 28 are controlled by a control voltage of a control node 34.
From a view from the LC tank, there is formed an impedance of a negative resistance on the differential amplifier, and consequently an output signal is generated from nodes 30 and 32 each connected to the drain of each MOSFET 10 and 12. The output signal generated from the nodes 30 and 32 is output at output nodes 18 and 24 through buffers 40 and 42 which include transistors.
However, as above described, in the conventional differential VCO, the buffer must be provided to pass the launch signal from the LC tank to other circuits, which results in much greater power consumption. That is, as shown in FIG. 1, since buffers 40 and 42 include transistors which consume additional power in addition to that consumed in each MOSFET embodying a negative resistance, much power is consumed.
Generally, it is very important to reduce power consumption in RF systems which use an oscillator and there are various methods for reducing the power consumption in the differential amplifier corresponding to a core portion of the oscillator. However, even if the power consumption in the differential amplifier decreases, the overall decrease in power consumption is limited since buffers 40 and 42 consume power continuously.
It is an object of the present invention to overcome the above described problem of conventional methods and therefore to provide a VCO using an LC tank and a differential amplifier which reduces power consumption in a buffer portion.
To achieve the above objects of the invention, there is provided a differential type voltage controlled oscillator comprising:
a current source;
a differential amplifier having a couple of transistors amplifying a difference of input signals controlled by the current source;
a couple of LC tanks oscillating an output signal from the transistors so as to generate the input signals; and
a couple of transformers utilizing an inductor of each LC tank as a primary coil and generating an oscillating output signal from a secondary coil thereof.
According to a preferred embodiment of the invention, there is also provided a capacitor connected in parallel or in series with the secondary coil of the transformer so as to remove a harmonic signal contained in the oscillating output signal.
The oscillating signal is output through the transformer (buffer) so that it does not consume additional power.